Erm, it's pretty simple solution, IMO.
If they're stacking health, you counter by getting spellvamp as LB. Single target spells get full use of spellvamp, and single target hits are what she excels at. Combined with Liandry's Torment (and Rylai's if you need to be semi-tanky to survive trades), and they'll regret stacking health.
A void staff/Liandry's is usually enough to counter hexdrinker. The idea is, you CAN counter Leblanc by building survivability, but doing so lowers your damage, and thereby making you useless. So yeah, there's going to be people who intentially build against you, so just outsmart them by using these items. You only deal average damage, but that's probably tons more than they can do.